This invention relates to a variable frequency response circuit and more particularly to such a circuit employed in a variable frequency generator, such as a signal generator, or a receiver.
The prior art is replete with a number of electric wave generators and radio receivers which may be digitally tuned and also provide a direct digital display of the frequency to which the device is tuned to or is responding to. Examples of such receivers and signal generators are many and many large corporations presently manufacture and sell both waveform generators and receivers employing direct digital display in connection with switch control tuning means.
Generally, both the receivers and the generators are designed to operate over a relatively wide frequency range and are varied in frequency over this range in coarse steps by utilizing a first frequency control means and may be varied continuously within each coarse step by employing a second frequency control means. In the case of radio receivers, certain receivers such as the DR 22 and the DR 33 as manufactured by McKay Dymek Co. of Pomona, Calif. employ a synthesizer to provide first local oscillator tuning in such a receiver. This approach is, of course, employed by many other manufacturers of similar devices.
The conventional synthesizer includes switching means which consist of a plurality of independent rotary switches. A first switch may be a multipositioned switch and operates to vary the frequency of a first local oscillator in 10 MHz steps. Hence, by setting this switch in one of its three positions, one may obtain a reading of zero, one, or two which is indicative of the lead digit in the 10 MHz band. A second switch is included which includes ten positions for varying the frequency of the first local oscillator in 1 MHz steps, as zero to nine. The third switch also includes ten positions and varies the frequency of the local oscillator in 100 KHz steps (zero to nine). A fourth switch includes twenty positions and varies the frequency of the first local oscillator in 10 KHz steps. The fourth switch also operates to vary the frequency of a second oscillator between a first frequency and a second higher frequency to provide 5 KHz steps. One can therefore tune the receiver in multidigit increments over a wide frequency range in 5 KHz intervals. Such a receiver, for example, may encompass a frequency range of operation from 50 KHz or lower to 30 MHz.
Hence, one can tune such a receiver by accessing the above described switches and rotating the same to a desired frequency. For example, if one desired to tune the receiver to a frequency of 27.41571 MHz, one would place switch 1 at position 2 indicative of 20 MHz. The second switch would be placed at position 7 indicative of 7 MHz. A third switch would be placed at position 4 indicative of 400 KHz and the fourth switch would be placed in the position indicative of 15 KHz. The tunable second oscillator enables tuning between 0 and 5 KHz to enable one to set the frequency to 27.41571 MHz and hence, to accurately specify the last two digits of the display. Thus, one would now read on the digital display associated with the receiver, the output frequency of 27.41571 MHz.
In such a receiver, a continuous tuning means is provided by a variable control. This continuous tuning means spans the 5 KHz intervals as specified by the fourth switch. This enables the user to perform fine tuning over the range indicated by the last two digits of the display to enable the user to accurately specify the frequency. This is especially important in responding to single sideband transmissions.
In such prior arrangements, it is necessary in tuning through a frequency range to first advance or retard the coarse frequency switch one position and then use the continuous tuning means to perform fine tuning from the low frequency end to the upper frequency end or vice versa.
Hence, as one can see from the above example, the continuous tuning means would operate to vary the frequency for example, from 27.41500 MHz to 27.42000 MHz. Now, if the coarse frequency control is changed, for example, by moving the position of the fourth switch to the next higher frequency, one would now have to move the continuous tuning means back to the low frequency end of its range and then move the same from its low frequency extremity back to the high frequency end. As one can ascertain, this procedure is extremely time consuming and inconvenient.
The continuous tuning control which is employed in such receivers is desirably a multiturn potentiometer to enable increased ease in receiving a single sideband and other modes which require very accurate frequency settings. Hence, as one can ascertain, the use of such a control in going from the low frequency end to the high frequency end and back again each time the coarse control is moved, is completely undesirable.
It is therefore an object of the present invention to provide a circuit arrangement especially useful in wide band tunable generators to enable continuous tuning of the same to permit response to any frequency signal located within the low and high frequency limits of each step available by the coarse tuning switches.
Still a further object of the invention is to provide a continuous tuning arrangement for a variable frequency response circuit of the type covering a relatively wide frequency range. The response circuit includes a first frequency determining means that alters the frequency of the circuit from a low to the high limit of the range in a discontinuous manner as by switches and a second frequency determining means that is adapted to change the frequency continuous over the range of each alteration; which second circuit is coupled with the first means to eliminate the discontinuity which would otherwise occur upon a change in frequency.